1. Field of the Invention
The invention relates to a method for producing syngas in combined operation with a metallurgical plant which comprises at least one blast furnace for producing crude iron, a converter steel mill and a coke-oven plant.
2. Description of the Related Art
Crude iron is obtained in the blast furnace from iron ores, additives such as coke and other reducing agents such as coal, oil, gas, biomasses, recycled waste plastics or other substances containing carbon and/or hydrogen. CO, CO2, hydrogen and water vapour inevitably occur as products of the reduction reactions. Apart from the aforementioned constituents, a blast-furnace top gas drawn off from the blast-furnace process often has a high content of nitrogen. The amount of gas and the composition of the blast-furnace top gas are dependent on the feedstock and the operating mode and are subject to fluctuations. Typically, however, blast-furnace top gas contains 35 to 60% by volume N2, 20 to 30% by volume CO, 20 to 30% by volume CO2 and 2 to 15% by volume H2. Around 30 to 40% of the blast-furnace top gas produced in the production of the crude iron is generally used for heating up the hot air for the blast-furnace process in air heaters; the remaining amount of top gas may be used in other areas of the mill for heating purposes or for electricity generation.
In the converter steel mill, which is arranged downstream of the blast-furnace process, crude iron is converted into crude steel. By blowing oxygen onto liquid crude iron, troublesome impurities such as carbon, silicon, sulphur and phosphorus are removed. Since the oxidation processes cause an intense development of heat, scrap is often added in amounts of up to 25% with respect to the crude iron as a coolant. Furthermore, lime is added for forming slag and an alloying agent. A converter gas that has a high content of CO and also contains nitrogen, hydrogen and CO2 is drawn off from the steel converter. A typical converter gas composition has 50 to 70% by volume CO, about 10 to 20% by volume N2, about 15% by volume CO2 and about 2% by volume H2. The converter gas is either burned off or, in the case of modern steel mills, captured and passed on to be used for providing energy.
In the coke-oven plant, coal is converted into coke by a coking process. A coke-oven gas thereby occurs, containing a high hydrogen content and considerable amounts of CH4. Typically, coke-oven gas contains 55 to 70% by volume H2, 20 to 30% by volume CH4, about 5 to 10% by volume N2 and about 5% by volume CO. In addition, the coke-oven gas has fractions of CO2, NH3 and H2S. In practice, the coke-oven gas is used in various areas of the works for heating purposes and in the power-generating process for electricity generation. In addition, it is known to use coke-oven gas together with blast-furnace top gas or with converter gas for producing syngases. According to a method known from WO 2010/136313 A1, coke-oven gas is separated into a hydrogen-rich gas stream and a residual gas stream containing CH4 and CO, the residual gas stream being fed to the blast-furnace process and the hydrogen-rich gas stream being mixed with blast-furnace top gas and processed further into a syngas. It is known from EP 0 200 880 A2 to mix converter gas and coke-oven gas and use them as a syngas for methanol synthesis.
In an integrated metallurgical plant that is operated in combination with a coking plant, approximately 40 to 50% of the raw gases that occur as blast-furnace top gas, converter gas and coke-oven gas are used for chemical engineering processes. Approximately 50 to 60% of the gases produced can be used for electricity generation or used as raw gases for producing syngas. By using the gases for producing syngas, the cost-effectiveness of a metallurgical plant can be improved. At the same time, the CO2 balance of the metallurgical plant also improves, since carbon is bound in chemical products and is not emitted in the form of CO2. However, it must be taken into consideration here that the amount of raw gas that can be used for producing syngas is subject to considerable fluctuations over time.
This is so because, as long as the raw gases are used for producing syngas, the production of electricity by a power-generating plant operated in combination with the metallurgical plant must be cut back and electricity obtained from external sources to cover the electricity demand of the metallurgical plant. If electricity is available at low cost and in sufficient amounts, for example from renewable energy sources, a great amount of raw gas can be used for producing syngas. On the other hand, when there is a high price for the externally obtained electricity, it is necessary from economic considerations to use the usable amount of raw gas that occurs in the metallurgical plant at least predominantly for electricity generation and to cut back the production of syngas.